[O'l Buck]

You might try going to your local GM dealer for the seal, they usually get the right part. or if you don't need to drive the truck to get the new seal, pull the pinion flange off and take the seal with you ( as long as you're lucky enough not to damage the part number on the seal taking it out). as mentioned above, mark the pinion flange and nut and count threads to make sure it goes back in the same place.